Additional plugs for PLW1000

> Can anyone tell me the procedure for adding an additional extender
> unit (the one with the two aerials) to the network?

   Have you looked at the documentation for these things?  Visit
http://netgear.com/support , put in your model number, and look for
Documentation.  Get the User Manual and/or the Installation Guide.
Read.  I see "Add Adapters" in the Installation Guide, and "Add an
Adapter to an Existing PowerLINE Network" in the User Manual, for
example.